1. Field of the Invention
The present invention relates to a display device using a self-emissive element such as an electroluminescence (hereinafter referred to as “EL”) element and a thin film transistor (hereinafter referred to as “TFT”).
2. Description of the Related Art
In the recent years, display devices using EL elements, which are self-emissive elements, have gained attention as the display devices that may replace CRTs and LCDs.
Research is being directed to development of display devices using TFTs as the switching elements for driving the EL elements.
FIG. 2 is an equivalent circuit diagram of a conventional organic EL display device. FIG. 3 is a plan view of the organic EL display device showing an area around a display pixel. FIG. 4A is a cross-sectional view taken along line A—A of FIG. 3, while FIG. 4B shows a cross-sectional view taken along line B—B of FIG. 3. FIG. 5 illustrates the arrangement of display pixels in the organic EL display device.
As shown in FIGS. 2 and 3, display pixels 110 are formed in regions surrounded by gate signal lines 51 and drain signal lines 52. The display pixels 110 are arranged in a matrix. A first TFT 30, which is a switching TFT, is provided in an area near an intersection of the two signal lines 51,52. The source 13s of the TFT 30 concurrently serves as a capacitor electrode 55 which functions together with a storage capacitor electrode line 54 described below to create capacitance. The source 13s is connected to the gate 41 of a second TFT 40 which is an EL element driving TFT. The source 43s of the second TFT 40 is connected to an anode 61 of an organic EL element 60. The drain 43d of the TFT 40 is connected to the power source line 53 which supplies a current to the organic EL element 60.
The storage capacitor electrode line 54 is arranged in parallel to gate signal lines 51. The storage capacitor electrode line 54 is made of a material such as chrome, and forms a storage capacitor 56 by allowing charges to be accumulated between the capacitor electrode 55 connected to the source 13s of the TFT 30 and the storage capacitor electrode line 54, while a gate insulating film 12 is disposed therebetween. The storage capacitor 56 is provided for retaining a voltage applied to the gate electrode 41 of the second TFT 40.
As shown in FIGS. 4A and 4B, the organic EL display device is formed by sequentially laminating TFTs and an organic EL element on a substrate 10 made of an insulating material such as glass or synthetic resin, or, alternatively, a conductive or semiconductor material. When a conductive or semiconductor substrate is used as the substrate 10, an insulating film such as SiO2 or SiN is formed on the substrate 10 before forming the first and second TFTs and the organic EL display element.
The first TFT 30, which is the switching TFT, is first described.
As shown in FIG. 4A, sequentially formed on an insulating substrate 10 made of a material such as quartz glass or non-alkali glass are an active layer 13 composed of a semiconductor film (p-Si film), a gate insulating film 12, and a gate signal line 51 made of a refractory metal such as chromium (Cr) or molybdenum (Mo). The gate signal line 51 concurrently serves as gate electrodes 11. Further arranged are a drain signal line 52 composed of Al, and a power source line 53 made of Al and serving as the drive power source of the organic EL element.
An interlayer insulating film 15 composed through sequential lamination of a SiO2 film, a SiN film, and a SiO2 film is formed on the entire surface over the gate electrodes 11 and the gate insulating film 12. Subsequently, a drain electrode 16 is formed by filling a metal such as Al in a contact hole created corresponding to the drain region 13d. A planarizing insulating film 17 composed of an organic resin is then formed over the entire surface for planarizing the surface.
The second TFT 40, which is the organic EL element driving TFT, is next described.
As shown in FIG. 4B, sequentially formed on an insulating substrate 10 made of a material such as quartz glass or non-alkali glass are an active layer 43 composed of a semiconductor film (p-Si film), a gate insulating film 12, and gate electrodes 41 made of a refractory metal such as Cr or Mo. The active layer 43 includes channels 43c, and source 43s and drain 43d on the outboard sides of the channels. An interlayer insulating film 15 composed through sequential lamination of a SiO2 film, a SiN film, and a SiO2 film is formed on the entire surface over the gate insulating film 12 and the active layer 43. Subsequently, a contact hole is created through the gate insulating film 12 and the interlayer insulating film 15 in a position corresponding to the drain region 43d. This contact hole is filled with a metal such as Al to connect the drain 43d with a power source line 53. The power source line 53 is connected to a drive power source. A planarizing insulating film 17 composed of, for example, an organic resin is then formed over the entire surface for planarizing the surface. A contact hole is formed through the planarizing insulating film 17 in a position corresponding to the source 43s. A transparent electrode made of ITO (indium tin oxide), namely, the anode 61 of the EL element 60, is formed on the planarizing insulating film 17 and through this contact hole to connect with the source 43s. 
The organic EL element 60 is configured by first providing the anode 61 made of a transparent electrode composed of a material such as ITO. The emissive element layer 66 is then superimposed. The emissive element layer 66 comprises a first hole-transport layer 62 composed of MTDATA or a similar material, a second hole-transport layer 63 composed of a material such as TPD, an emissive layer 64 composed of, for example, Bebq2 including quinacridone derivatives, and an electron transport layer 65 composed of a material such as Bebq2. Subsequently, the cathode 67 is formed, for example, by using an alloy of magnesium (Mg) and silver (Ag), or by laminating lithium fluoride (LiF) and Al. All of the above-mentioned layers constituting the organic EL element 60 are laminated in the described order. By selecting materials which emit light of predetermined colors as the emissive materials of the emissive element layer 66, display pixels can be configured to emit light of different colors. The organic EL display device is configured by arranging the display pixels of respective colors in a matrix layout.
In the organic EL element, holes injected from the anode and electrons injected from the cathode recombine in the emissive layer. As a result, organic molecules constituting the emissive layer are excited, generating excitons. Through the process in which these excitons undergo radiation until deactivation, light is emitted from the emissive layer. This light radiates outward through the transparent anode via the transparent insulating substrate, resulting in light emission.
As described above, in each display pixel 110 emitting a color, an EL element driving TFT 40 is connected for driving the organic EL element of that pixel. FIG. 3A is an enlarged view of a driving TFT 40. The transistor size of the TFT 40, namely, ratio W/L concerning the channel width W and the channel length L of the channel region in which the semiconductor film of the TFT 40 and the gate electrodes overlap (in the case of FIG. 3A, L=L1+L2) is identical in all TFTs 40.
Furthermore, emissive efficiency of the emissive layer in each display pixel differs according to the emitted color depending on the organic emissive material constituting the emissive layer.
In such an arrangement, to supply different values of current to the organic EL elements for different colors according to the emissive efficiency of each color so as to obtain the same level of luminance for the respective colors and establish an appropriate white balance, it is necessary to alter the current value of the power source for each color, or to alter the potential of the drain signal supplied to the first TFT connected in each display pixel according to each color. Specifically, more current must be made to flow in an organic EL element including an emissive layer of a color having a low emissive efficiency, compared to an organic EL element including an emissive layer of a color having a high emissive efficiency.
However, conventional display devices are disadvantageous in that, to alter the current value of the power source for each color of the display pixels, the power source line must be arranged in a complex manner within the region in which the display pixels are arranged. Further, to alter the potential of the drain signal supplied to the first TFT according to each color, complex circuitry is necessary for supplying a signal to the first TFT.